There is ever-present pressure in the semiconductor industry to develop smaller and more highly integrated devices. As the demand increases for cheaper, faster, less power consuming yet more powerful integrated circuits such as microprocessors and memory devices, the transistor packing density of the integrated circuit must be similarly increased.
As the industry standard approaches smaller and smaller scaled devices, problems with further advancement are presented and it becomes more difficult to produce sub-micron devices that can perform as desired. For example, in producing memory systems for such increasingly compact devices, the goal of expanding or, at least, maintaining memory capacity as cell size shrinks must be attained without resorting to processes that increase the number of masking, deposition, etch and other steps in the production process. In particular, any time a photomasking layer is applied, a subsequent step will also be required, either implant or etch. These additional steps add significantly to manufacturing costs. High capital costs are associated with photolithographic equipment and more complex photo processing, in terms of more photo process steps per level, more equipment, and the use of expensive ultra clean room floor space.
Furthermore, the smallest separation between semiconductor structures using known patterning methods is subject to the physical limitations of how thin a distinguishable line or gap can be formed in a photoresist layer by patterning with a reticle. Thus, use of such photolithographic patterning methods places an upper limit to the highest achievable density of a semiconductor device. In addition, defect density is inevitably increased with each additional photomasking layer, which thereby compromises yield and reliability.
In light of these and other difficulties, the semiconductor industry is in need of a method of fabricating high density semiconductor devices without increasing, or even reducing the use of photolithographic patterning processes to produce the device. Along the same lines, it is also desirable to increase the density of existing memory devices which at least maintains, or perhaps even increases, the memory capacity of the memory cells in the device.